The detection of exoplanets orbiting other stars has revolutionized our view of the cosmos. First results suggest that it is teeming with a fascinating diversity of rocky planets, including those in ...the habitable zone. Even our closest star, Proxima Centauri, harbors a small planet in its habitable zone, Proxima b. With the next generation of telescopes, we will be able to peer into the atmospheres of rocky planets and get a glimpse into other worlds. Using our own planet and its wide range of biota as a Rosetta stone, we explore how we could detect habitability and signs of life on exoplanets over interstellar distances. Current telescopes are not yet powerful enough to characterize habitable exoplanets, but the next generation of telescopes that is already being built will have the capabilities to characterize close-by habitable worlds. The discussion on what makes a planet a habitat and how to detect signs of life is lively. This review will show the latest results, the challenges of how to identify and characterize such habitable worlds, and how near-future telescopes will revolutionize the field. For the first time in human history, we have developed the technology to detect potential habitable worlds. Finding thousands of exoplanets has taken the field of comparative planetology beyond the Solar System.
Most models used to predict or fit exoplanet transmission spectra do not include all the effects of atmospheric refraction. Namely, the angular size of the star with respect to the planet can limit ...the lowest altitude, or highest density and pressure, probed during primary eclipses as no rays passing below this critical altitude can reach the observer. We discuss this geometrical effect of refraction for all exoplanets and tabulate the critical altitude, density, and pressure for an exoplanet identical to Earth with a 1 bar N sub(2)/O sub(2) atmosphere as a function of both the incident stellar flux (Venus, Earth, and Mars-like) at the top of the atmosphere and the spectral type (O5-M9) of the host star. We show that such a habitable exo-Earth can be probed to a surface pressure of 1 bar only around the coolest stars. We present 0.4-5.0 mu m model transmission spectra of Earth's atmosphere viewed as a transiting exoplanet, and show how atmospheric refraction modifies the transmission spectrum depending on the spectral type of the host star. We demonstrate that refraction is another phenomenon that can potentially explain flat transmission spectra over some spectral regions.
A Volcanic Hydrogen Habitable Zone Ramirez, Ramses M.; Kaltenegger, Lisa
Astrophysical journal. Letters,
03/2017, Letnik:
837, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The classical habitable zone (HZ) is the circular region around a star in which liquid water could exist on the surface of a rocky planet. The outer edge of the traditional N2-CO2-H2O HZ extends out ...to nearly ∼1.7 au in our solar system, beyond which condensation and scattering by CO2 outstrips its greenhouse capacity. Here, we show that volcanic outgassing of atmospheric H2 can extend the outer edge of the HZ to ∼2.4 au in our solar system. This wider volcanic-hydrogen HZ (N2-CO2-H2O-H2) can be sustained as long as volcanic H2 output offsets its escape from the top of the atmosphere. We use a single-column radiative-convective climate model to compute the HZ limits of this volcanic hydrogen HZ for hydrogen concentrations between 1% and 50%, assuming diffusion-limited atmospheric escape. At a hydrogen concentration of 50%, the effective stellar flux required to support the outer edge decreases by ∼35%-60% for M-A stars. The corresponding orbital distances increase by ∼30%-60%. The inner edge of this HZ only moves out ∼0.1%-4% relative to the classical HZ because H2 warming is reduced in dense H2O atmospheres. The atmospheric scale heights of such volcanic H2 atmospheres near the outer edge of the HZ also increase, facilitating remote detection of atmospheric signatures.
The habitable zone (HZ) is the circumstellar region where standing bodies of liquid water could exist on the surface of a rocky planet. Conventional definitions assume that CO2 and H2O are the only ...greenhouse gases. The outer edge of this classical N2-CO2-H2O HZ extends out to nearly ∼1.7 au in our solar system, beyond which condensation and scattering by CO2 outstrip its greenhouse capacity. We use a single-column radiative-convective climate model to assess the greenhouse effect of CH4 (10-∼100,000 ppm) on the classical HZ (N2-CO2-H2O) for main-sequence stars with stellar temperatures between 2600 and 10,000 K (∼A3 to M8). Assuming N2-CO2-H2O atmospheres, previous studies have shown that cooler stars heat terrestrial planets more effectively. However, we find that the addition of CH4 produces net greenhouse warming (tens of degrees) in planets orbiting stars hotter than a mid-K (∼4500 K), whereas a prominent anti-greenhouse effect is noted for planets around cooler stars. We show that 10% CH4 can increase the outer edge distance of the hottest stars (TEFF = 10,000 K) by over 20%. In contrast, the CH4 anti-greenhouse can shrink the HZ for the coolest stars (TEFF = 2600 K) by a similar percentage. We find that dense CO2-CH4 atmospheres near the outer edge of hotter stars may suggest inhabitance, highlighting the importance of including secondary greenhouse gases in alternative definitions of the HZ. We parameterize the limits of this N2-CO2-H2O-CH4 HZ and discuss implications in the search for extraterrestrial life.
HABITABLE ZONES OF POST-MAIN SEQUENCE STARS Ramirez, Ramses M.; Kaltenegger, Lisa
Astrophysical journal/The Astrophysical journal,
05/2016, Letnik:
823, Številka:
1
Journal Article
Recenzirano
Odprti dostop
ABSTRACT Once a star leaves the main sequence and becomes a red giant, its Habitable Zone (HZ) moves outward, promoting detectable habitable conditions at larger orbital distances. We use a ...one-dimensional radiative-convective climate and stellar evolutionary models to calculate post-MS HZ distances for a grid of stars from 3700 to 10,000 K (∼M1 to A5 stellar types) for different stellar metallicities. The post-MS HZ limits are comparable to the distances of known directly imaged planets. We model the stellar as well as planetary atmospheric mass loss during the Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) phases for super-Moons to super-Earths. A planet can stay between 200 million years up to 9 Gyr in the post-MS HZ for our hottest and coldest grid stars, respectively, assuming solar metallicity. These numbers increase for increased stellar metallicity. Total atmospheric erosion only occurs for planets in close-in orbits. The post-MS HZ orbital distances are within detection capabilities of direct imaging techniques.
ABSTRACT
The TRAPPIST-1 system is a priority target for terrestrial exoplanet characterization. TRAPPIST-1e, residing in the habitable zone, will be observed during the James Webb Space Telescope ...(JWST) GTO Program. Here, we assess the prospects of differentiating between prebiotic and modern Earth scenarios for TRAPPIST-1e via transmission spectroscopy. Using updated TRAPPIST-1 stellar models from the Mega-MUSCLES survey, we compute self-consistent model atmospheres for a 1 bar prebiotic Earth scenario and two modern Earth scenarios (1 and 0.5 bar eroded atmosphere). Our modern and prebiotic high-resolution transmission spectra ($0.4\!-\! 20\, \rm{\mu m}$ at R ∼100 000) are made available online. We conduct a Bayesian atmospheric retrieval analysis to ascertain the molecular detectability, abundance measurements, and temperature constraints achievable for both scenarios with JWST. We demonstrate that JWST can differentiate between our prebiotic and modern Earth scenarios within 20 NIRSpec Prism transits via CH4 abundance measurements. However, JWST will struggle to detect O3 for our modern Earth scenario to $\gt 2\, \sigma$ confidence within the nominal mission lifetime (∼ 80 transits over 5 yr). The agnostic combination of N2O and/or O3 offers better prospects, with a predicted detection significance of $2.7\, \sigma$ with 100 Prism transits. We show that combining MIRI LRS transits with Prism data provides little improvement to atmospheric constraints compared to observing additional Prism transits. Though biosignatures will be challenging to detect for TRAPPIST-1e with JWST, the abundances for several important molecules – CO2, CH4, and H2O – can be measured to a precision of ≲ 0.7 dex (a factor of 5) within a 20 Prism transit JWST program.
ABSTRACT
Our first targets in the search for signs of life are orbiting nearby M stars, such as the planets in the Proxima Centauri, Ross-128, LHS-1140, and TRAPPIST-1 systems. Future ground-based ...discoveries, and those from the TESS mission, will provide additional close-by targets. However, young M stars tend to be very active, flaring frequently and causing UV fluxes on the surfaces of HZ planets to become biologically harmful. Common UV-protection methods used by life (e.g. living underground, or underwater) would make a biosphere harder to detect. However, photoprotective biofluorescence, ‘up-shifting’ UV to longer, safer wavelengths, could increase a biosphere's detectability. Here we model intermittent emission at specific wavelengths in the visible spectrum caused by biofluorescence as a new temporal biosignature for planets around active M stars. We use the absorption and emission characteristics of common coral fluorescent pigments and proteins to create model spectra and colours for an Earth-like planet in such a system, accounting for different surface features, atmospheric absorption, and cloud cover. We find that for a cloud-free planet biofluorescence could induce a temporary change in brightness that is significantly higher than the reflected flux alone, causing up to two orders-of-magnitude change in planet–star contrast, compared to a non-fluorescent state, if the surface is fully covered by a highly efficient fluorescent biosphere. Hence, UV-flare induced biofluorescence presents previously unexplored possibilities for a new temporal biosignature that could be detectable by instruments like those planned for the extremely large telescope and could reveal hidden biospheres.
An Earth-like exoplanet orbiting a white dwarf (WD) would be exposed to different UV environments than Earth, influencing both its atmospheric photochemistry and UV surface environment. Through the ...use of a coupled 1D climate-photochemistry code, we model atmospheres of Earth-like planets in the habitable zone (HZ) of WDs for surface temperatures between 6000 and 4000 K, corresponding to about 7 billion years of WD evolution, and discuss the evolution of planetary models in the HZ during that evolution.
ABSTRACT
Ground- and space-based planet searches employing radial velocity techniques and transit photometry have detected thousands of planet-hosting stars in the Milky Way. With so many planets ...discovered, the next step toward identifying potentially habitable planets is atmospheric characterization. While the Sun–Earth system provides a good framework for understanding the atmospheric chemistry of Earth-like planets around solar-type stars, the observational and theoretical constraints on the atmospheres of rocky planets in the habitable zones (HZs) around low-mass stars (K and M dwarfs) are relatively few. The chemistry of these atmospheres is controlled by the shape and absolute flux of the stellar spectral energy distribution (SED), however, flux distributions of relatively inactive low-mass stars are poorly understood at present. To address this issue, we have executed a panchromatic (X-ray to mid-IR) study of the SEDs of 11 nearby planet-hosting stars, the
Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems
(MUSCLES) Treasury Survey. The MUSCLES program consists visible observations from
Hubble
and ground-based observatories. Infrared and astrophysically inaccessible wavelengths (EUV and Ly
α
) are reconstructed using stellar model spectra to fill in gaps in the observational data. In this overview and the companion papers describing the MUSCLES survey, we show that energetic radiation (X-ray and ultraviolet) is present from magnetically active stellar atmospheres at all times for stars as late as M6. The emission line luminosities of C
iv
and Mg
ii
are strongly correlated with band-integrated luminosities and we present empirical relations that can be used to estimate broadband FUV and XUV (≡X-ray + EUV) fluxes from individual stellar emission line measurements. We find that while the slope of the SED, FUV/NUV, increases by approximately two orders of magnitude form early K to late M dwarfs (≈0.01–1), the absolute FUV and XUV flux levels at their corresponding HZ distances are constant to within factors of a few, spanning the range 10–70 erg cm
−2
s
−1
in the HZ. Despite the lack of strong stellar activity indicators in their optical spectra, several of the M dwarfs in our sample show spectacular UV flare emission in their light curves. We present an example with flare/quiescent ultraviolet flux ratios of the order of 100:1 where the transition region energy output during the flare is comparable to the total quiescent luminosity of the star
E
flare
(UV) ∼ 0.3
L
*
Δ
t
(Δ
t
= 1 s). Finally, we interpret enhanced
L
(line)/
L
Bol
ratios for C
iv
and N
v
as tentative observational evidence for the interaction of planets with large planetary mass-to-orbital distance ratios (
M
plan
/
a
plan
) with the transition regions of their host stars.
Abstract
Reflection spectroscopy holds great promise for characterizing the atmospheres and surfaces of potentially habitable terrestrial exoplanets. The surface of the modern Earth exhibits a sharp ...albedo change near 750 nm caused by vegetation—the
red edge
—which would leave a strong spectral signature if present on an exoplanet. However, the retrieval of wavelength-dependent surface properties from reflection spectra has seen relatively little study. Here, we propose a new surface albedo parameterization capable of retrieving the wavelength location of a priori unknown “edge-like” features. We demonstrate that a wavelength-dependent surface albedo model achieves higher accuracy in retrieving atmospheric composition than a uniform albedo model. Wavelength-dependent surfaces are also generally preferred over a uniform albedo model when retrieving simulated reflection spectra for a modern Earth analog, even for moderate signal-to-noise ratios (S/N = 10) and Earth-like clouds. Further, the location of the modern Earth’s red edge can be robustly and precisely constrained (within 70 nm for S/N = 10). Our results suggest that future space-based direct-imaging missions have the potential to infer surface compositions for rocky exoplanets, including spectral edges similar to those caused by life on the modern Earth.
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